Businesses or other entities store their operational data (e.g., customer lists, financial transactions, business documents, business transactions, etc.) in logical volumes on memory devices such as hard disks. Unfortunately, the operational data is susceptible to corrupting events such as hardware failure, software failure, or user error. Hardware failures can range from the failure of a single hard disk to the destruction of an entire data center containing the hard disk, making some or all of the business's operational data unrecoverable. Software failures are procedural errors in an application that corrupt operational data. User errors include errors such as inadvertent deletion or overwriting of operational data that is later required. Failures and user errors often result in the loss or corruption of important operational data. The loss or corruption of operational data is particularly devastating to a business that relies heavily on electronic commerce.
Recognizing the importance of maintaining reliable operational data, businesses or other entities typically employ backup systems to protect critical operational data against corruption. Backup systems create backup or point-in-time (PIT) copies of operational data. A backup copy captures an image of the operational data at an instant in time so that if need be, everything that has happened to the operational data since that instant can be forgotten, and the state of operations (as reflected in the operational data) can be restored to that instant. Relatively simple backup programs can be used that interface with computer operating systems to backup data (e.g., files and directories stored on a hard disk) to magnetic tapes and to restore such data from the tapes onto the hard disk. Backup programs can run on standalone computer systems or on a server within a network of computer systems.
While it is a simple and fairly quick procedure to create a backup copy of an individual data object (image file, text file, etc.) the creation of a backup copy becomes more difficult as the amount of operational data increases. One method of creating a backup copy of a large volume of operational data is to copy the data from the hard disks that store the volume to one or more magnetic tapes. Once the backup has completed, the magnetic tapes are stored either locally or remotely. When a data corruption is detected in the operational data volume as a result of hardware failure, software failure, or user error, the volume is restored to its pre-corrupted data state using the backup copy.
Backup operations create backup copies that may be either full or incremental. A full backup copy means that all files in the data volume are copied regardless of how recently the files have been modified or whether a previous backup copy exists. An incremental backup means that only files of the volume that have changed since some previous event (e.g., a prior full backup or prior incremental backup) are copied. The backup window (the time needed to create the backup copy) for a full backup operation tends to be much larger when compared to the backup window for an incremental backup operation. For most applications, incremental backups are preferable at backup times since, in most cases, the number of files of the data volume that change between backups is very small compared to the number of files in the entire data volume and since the backup window is small. If backups are done daily or even more frequently, it is not uncommon for less than 1% of files of a volume to change between backups. An incremental backup in this case copies 1% of the data that a full backup would copy and uses 1% of the input/output (IO) resources between the hard disks and the backup magnetic tapes.
Incremental backup appears to be the preferred mode of protecting data. And so it is, until a full restore of all the files of the data volume is needed. A full restore from incremental backups requires starting with a restore using the newest full backup copy, followed by restores of all newer incremental backups. That can require a lot of magnetic tape handling performed by, for example, an automated robotic handler.
Restore from full backups is genuinely simpler and more reliable than restores from combinations of full and incremental backups. When recovering from individual user errors, the situation is just the opposite. Users tend to work with one small set of files for a period of days or weeks and then work with a different set. Accordingly, there is a high probability that a file erroneously corrupted by a user will have been used recently and therefore will be copied in one of the incremental backups. Since incremental backups contain a smaller fraction of data when compared to a full backup, the incremental backups can usually be searched much faster for the target file if a restore is required. From the individual user's standpoint, it is preferable to create many small incremental backups.
While backup is useful, it presents a number of disadvantages. The applications or database management systems that access volumes are required to be in “hot backup mode” before a backup operation starts on a data volume to ensure that the resulting back up copy is consistent with the data volume. Hot backup mode can impact the performance of applications or database management systems. For example, applications or database management systems cannot write new data to the volume until the backup operation has completed since the data contents of the volumes must remain unchanged during the backup window. Otherwise the resulting backup copy will not be consistent to a single point in time. A data storage log can be employed to store new volume data until the backup operation completes, and the volumes can be overwritten with the data stored in the storage log when the backup operation completes. However, these storage logs are complicated. Another problem is that applications often require that the application's backup method be used for backups. This limits the ability of an independent backup product to provide backup services without coordination with the application.